The “Print Clock” is a system of dating prints developed by S. Blair Hedges, Professor of Biology at Penn State. The method depends on the supposition that printing surfaces deteriorated at a clock-like rate.

The print clock is a method for dating undated books and prints that were produced by hand-operated presses, thus since the 15th Century. It is based on the hypothesis, backed by observations, that woodblocks and metal plates (e.g., copperplates) deteriorated in a clock-like manner during their lifespan, which was often decades… By measuring this change across different editions of the same print, and calibrating the rate with dated editions, it is possible to estimate the age of the undated work.

From a printmaker’s perspective, the most interesting thing about this (aside from these sweet diagrams) is the conclusions Hedges has drawn about how printing matrices become worn.

Aging of the wood creates breaks in the relief of a carved woodblock, causing line breaks on the resulting print. For copperplates, the image fading I measured is the result of the thinning of the etched and engraved lines caused by the erosion of the copper surface, which results from the steady corrosive effects of acid in the atmosphere plus the periodic removal of the accumulated corrosion by scouring and polishing of the plates prior to each print run… The assumption in published papers in this field was that prints from later editions of the same copperplate are paler because the enormous pressures in the printing process were flattening the copperplate, but my laboratory studies proved that this is not the case.

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